1. Field of the Invention
The present invention relates to a photo-catalyzer which utilizes a photo-catalytic reaction by means of a photo-catalyst including, for instance, a titanium dioxide film. A photo-catalyzer according to the present invention is capable of cleaning and deodorizing, destroying or repelling micro-organisms, and including undertaking air or water (e.g., drinking water, beverage) purification.
2. Description of Related Art
A photo-catalytic reaction, especially a strong oxidizing catalytic reaction, is of recent interest to engineers. The photo-catalytic reaction can be effected by fine particles of photo-semiconductor material, for example titanium dioxide (TiO.sub.2).
When a particulate photo-semiconductor substance, such as titanium dioxide, is irradiated by light having at least the band-gap energy of the photo-semiconductor material (i.e., when titanium dioxide is irradiated by ultraviolet light having a wavelength of 400 nm or less), electrons present in the valency electron band are excited to migrate to the conduction band. Thus, free electrons are generated in the conduction band, and at the same time positively-charged particles (i.e., positive holes) are generated in the valency band. These positive holes and free electrons move in the semiconductor particulates, and later recombine over time. However, when there exists air, water, or compounds and ions in the substance having a level width (i.e., the spread in energy of an unstable state, equal to the difference between the energies at which intensity of emission or absorption of particles is one-half maximum value) lower than the energies of the positive holes and free electrons on the surface of the particulates, the positive holes and free electrons migrate into the compounds and ions by way of the particulate surface.
As a result, the positive holes directly oxidize the compounds and ions present on the particulate surfaces, or produce hydroxide-group radicals, one form of activated oxygen. The free electrons cause reduction reactions; mainly to reduce oxygen to reactive oxygen species, i.e., the free electrons add to oxygen to produce oxygen species having an oxidizing capability. Thus, when light is irradiated onto the fine particulates of photo-semiconductor, the fine particulates form an oxidative activated surface to act as a catalyst for the decomposition, or the like, of organic compounds. These operations of photo-semiconductor fine particulates are described in an article "Photo-related Catalytic Chemistry" in "KAGAGU SOSETSU (Chemistry Outline)", a Japanese quarterly magazine, No. 23, 1994.
Among photo-semiconductors, titanium dioxide exhibits an extremely high oxidizing catalytic action when used in fine particulate form. Titanium dioxide is also superb in terms of stability and safety. Many applications of titanium dioxide are known. Among known applications, titanium dioxide may be processed to a fine powder, and the fine powder may be applied as a film on a surface of a substrate to constitute a photo-catalyst. When the photo-catalyst is irradiated by ultraviolet light, it exhibits a high oxidizing capability which can be utilized to decompose organic compounds, etc.
Hollow glass beads, coated with a photo-catalyst comprising a titanium dioxide film, could be used as an agent for decomposing crude oil spilled on the sea. The crude oil deposited on the surface of the glass beads is decomposed by the strong oxidizing catalytic action of titanium dioxide which is activated by the ultraviolet component of sunlight.
Photo-catalysts, such as titanium dioxide, could be used to deodorize or destroy smells in indoor air, to destroy or repel bacteria in indoor air, or to decompose dirt-like cigarette tar or oil films. In these applications, the photo-catalyst is activated by utilizing ultraviolet radiation contained in natural light or light emitted from fluorescent lamps. The oxidizing catalytic action of the activated photo-catalyst is utilized to decompose odor compounds, such as mercaptan, or organic compounds, such as cigarette tar, or to destroy micro-organisms, such as fungi or bacteria, or to inhibit micro-organisms from growing.
Sunlight or fluorescent lamps have limitations when used as a source of ultraviolet light to activate a photo-catalyst comprising a titanium dioxide film, or the like. Accordingly, the photo-catalyst cannot be used where no sunlight shines or where no large-capacity fluorescent lamp can be provided to activate the photo-catalyst.